I have about a half dozen day lilies struggling this year. Most of them are ones that exploded on me last year and were phenomenal. Then we had a very cold winter, and I thought I lost them. But slowly they are coming back. They won't bloom, look like seedlings.
For me, I am just going to put it down as stress, give them a little extra TLC, and leave them alone. I will probably make sure they stay well mulched this fall / winter and see what happens next year. ~J